1. Field of the Invention
The present invention relates to a power supply device that supplies power to an electrical load such as a motor, and a method for driving the same.
2. Description of the Related Art
(Explanation of a Power Supply Device for which the Present Invention is Useful)
A known example of a power supply device is shown in FIGS. 8A to 8F. A power supply device 80 of FIG. 8A is a device for supplying power to a motor M. The power supply device 80 is provided with a series circuit A in which two switching elements A1 and A2 are connected in series and a series circuit B in which two switching elements B1 and B2 are connected in series. The series circuits A and B are connected in parallel. The aforesaid parallel circuit is connected between a pair of terminals, c and d, of a direct current power source S. An intermediate electric potential point a between the switching elements A1 and A2 of the series circuit A is connected to one of the power supply points of the motor M. An intermediate electric potential point b between the switching elements B1 and B2 of the series circuit B is connected to the other power supply point of the motor M.
FIG. 8B shows a state in which power is supplied to the motor M, wherein the switching elements B1 and A2 are in an on-state, and the switching elements A1 and B2 are in an off-state. In this case, current is supplied to the motor M in the direction indicated by an arrow. FIG. 8E shows another state in which power is supplied to the motor M, wherein the switching elements A1 and B2 are in the on-state, and the switching elements B1 and A2 are in the off-state. In this case, current is supplied to the motor M in the direction indicated by an arrow. The power supply device 80 is able to switch the direction in which the power is supplied to the motor M.
FIG. 8A shows an intermediate stage of switching from FIG. 8B to FIG. 8E. In FIG. 8A, the switching element A2 that had been in the on-state in FIG. 8B is switched to the off-state. Subsequently, as a result of the switching element B1 being switched off and the switching elements A1 and B2 being switched on, the state is switched to the state shown in FIG. 8B. The states of those switching elements that are switched between the on-state and off-state are indicated in boxes to facilitate easy understanding.
Upon switching from the state shown in FIG. 8B to the state shown in FIG. 8A when the current flowing to the motor M suddenly becomes zero, a high voltage is generated due to a reactance component of the motor M. It is possible that this high voltage destroys the switching elements A1, A2, B1 and B2 by acting thereon. In order to avoid this, a diode is connected in reverse parallel to the switching element A1. When a diode is connected in reverse parallel to the switching element A1, the motor current continues to flow as indicated by an arrow of FIG. 8A, thereby making it possible to prevent the high voltage from acting on the switching elements A1, A2, B1 and B2. The diode connected in reverse parallel to the switching element is referred to as a return diode, and current that continues to flow even after the switching element A2 has switched to the off-state is referred to as a return current. The return diode is required to have a low forward voltage drop.
As shown in FIG. 8A, when the state of FIG. 8E is realized by switching the switching element A1 to the on-state under a condition in which the return current is flowing to the return diode connected in reverse parallel to the switching element A1, a reverse recovery current flows to the return diode to which the aforesaid return diode has flown. When a large reverse recovery current flows, a high voltage is imposed on the switching elements A1, A2, B1 and B2, resulting in the possibility of damage thereof. A technology is required that reduces the reverse recovery current that flows to the return diode to a low level.
The description above has provided an explanation of the return current that flows when the state of FIG. 8B is changed to the state of FIG. 8E through the state of FIG. 8A. However, situations in which the return current flows are not limited thereto.
A technology is known for regulating a root-mean-square (RMS) current supplied to the motor M by repeatedly switching between the state of FIG. 8B and the state of FIG. 8A. If the duration of the state of FIG. 8B is long and the duration of the state of FIG. 8A is short, a large current passes through the motor M. If the duration of the state of FIG. 8B is short and the duration of the state of FIG. 8A is long, a small current passes through the motor M. The return current also flows when realizing the state in FIG. 8A in order to control electric current.
RMS current supplied to the motor M can be adjusted sinusoidally by repeatedly switching between the state of FIG. 8B and the state of FIG. 8A. The direction of current supplied to the motor M can be inverted by switching between the state of FIG. 8B and the state of FIG. 8E. Combination of the aforesaid two makes it possible to apply alternating current to the motor M. The power supply device 80 can also be said as a conversion device that converts a direct current to an alternating current.
When switching from the state of FIG. 8A to the state of FIG. 8B, the reverse recovery current flows to the return diode to which the return current had previously been flowing. When such a large reverse recovery current flows, a high voltage acts on the switching elements A1, A2, B1 and B2 resulting in the possibility of the destruction thereof. A technology is required that reduces reverse recovery current flowing to the return diode to a low level.
In the case of switching the current direction by switching from the state of FIG. 8B to the state of FIG. 8E, the state of FIG. 8B may be switched to the state of FIG. 8E via the state of FIG. 8C. Switching between FIG. 8B and FIG. 8C may alternately be repeated to control the electric current. In the case of FIG. 8C, the return current flows to a return diode connected in reverse parallel to the switching element B2.
In the case of changing the current direction by switching from the state of FIG. 8E to the state of FIG. 8B, FIG. 8E may be switched to FIG. 8B via the state of FIG. 8D; alternately, FIG. 8E may be switched to FIG. 8B via FIG. 8F. In order to control the electric current, FIG. 8E and FIG. 8D may be switched repeatedly, and FIG. 8E and FIG. 8F may alternately be switched repeatedly. In the case of FIG. 8D, the return current flows to a return diode connected in reverse parallel to the switching element B1, and in the case of FIG. 8F, the return current flows to a return diode connected in reverse parallel to the switching element A2.
In any case, the return diode is required to have a low forward voltage drop, and it is necessary to reduce the reverse recovery current that flows to the return diode to a low level.
FIGS. 9A to 9I, 10A to 10I and 11A to 11I indicate examples of power supply devices for a three-phase motor. In each of these power supply devices, phase of current passing through the motor M can be switched by switching in the order of B, E and H of each drawing. During switching of the phase, the state switches to the state of A or C, D or F, or G or I of each drawing. Alternatively, the state switches to the state of either: A or C, D or F, or G or I of each drawing to adjust the RMS current magnitude that passes through the motor M. When switching to the state of A or C, D or F, or G or I of each drawing by switching off a switching element that had previously been on, the return current flows to the return diode. The switching elements are protected from the high voltage acting thereon by directing the return current to flow to the return diode. In any of these power supply devices, since the return current flows to the return diode when switching to the either state of A or C, D or F, or G or I as in the respective drawing, the return diode is required to have a low forward voltage drop. Since the reverse recovery flows to the return diode in any of the power supply devices when switching to either state of B, E or H from A or C, D or F, or G or I of each drawing, it is necessary to reduce the reverse recovery order to a low level.
In any of the power supply devices of FIGS. 9A to 9I, 10A to 10I and 11A to 11I, a three-phase alternating current is provided to the motor M by combining switching of the current direction and adjustment of the RMS current. All of these power supply devices are conversion devices that convert direct current to the three-phase alternating current.
All of the power supply devices shown in FIGS. 8A to 8F, 9A to 9I, 10A to 10I and 11A to 11I are each provided with switching elements and composition circuits connected in reverse parallel to the switching element. In each of these power supply devices, the plurality of composition circuits are connected in series, and a plurality of such series circuits are connected in parallel. The aforesaid parallel circuit is connected between a pair of power supply terminals, and intermediate electric potential points between the composition circuits of each series circuit are connected to a load. The power supply devices provide electric power from a power source to the load. The power supply devices switch the direction in which power is supplied to the load, or adjust the amount of the RMS current supplied to the load.
The plurality of switching elements switches states according to the following rules:
(1) a switching element on one side of an intermediate electric potential point of one series circuit is switched to the on-state;
(2) a switching element on another side of the intermediate electric potential point of the series circuit of (1) above is switched to the off-state;
(3) a switching element on one side of an intermediate electric potential point of at least one of the other series circuits is switched to the off-state; and,
(4) a switching element on another side of the intermediate electric potential point of the series circuit of (3) above is switched to the on-state, so that electric power is supplied from the power supply to the load via the two switching elements switched to the on-state of (1) and (4).
In the case of FIG. 9B, for example, C1 is switched on according to (1), C2 is switched off according to (2), both A1 and B1 are switched off according to (3), and both A2 and B2 are switched on according to (4). In the case of FIG. 10B, C1 and B1 are switched on according to (1), C2 and B2 are switched off according to (2), A1 is switched off according to (3), and A2 is switched on according to (4). In the case of FIG. 11B, C1 is switched on according to (1), C2 is switched off according to (2), A1 is switched off according to (3), and A2 is switched on according to (4). In the case of FIG. 11, both B1 and B2 are switched off according to (1), (2) and (3) respectively.
In the case of FIGS. 9 and 10, one switching element can be in the on-state on one side while two switching elements can be switched to the on-state on another side, or two switching elements can be in the on-state on one side while one switching element is switched to the on-state on another side. As shown in FIG. 11, the electric power can be supplied to the load if a switching element on aforesaid another side is switched on in at least one series circuit that differs from a series circuit in which a switching element on one side is switched on.
In this type of power supply device, the direction in which electric power is supplied to the load can be sequentially switched by sequentially changing the series circuit in which a switching element on one side is to be switched to the on-state according to (1) above. In the case of FIGS. 9 and 10, a rotating magnetic field can be created in the three-phase motor M by switching in an order of B, E and H.
When a switching element having been switched to the on-state according to (1) above is switched to the off-state, the return current flows to the return diode connected in reverse parallel to a switching element according to (2) above.
In the case of FIG. 8, when A2 switched on in FIG. 8B is switched off, as to the state of FIG. 8A, the return current flows to the return diode connected in reverse parallel to the switching element A1 as in (2) above. If B1 switched on in FIG. 8B is switched off to the state of FIG. 8C, the return current flows to the return diode connected in reverse parallel to the switching element B2 as in (2) above.
In the case of FIG. 9, when C1 switched on in FIG. 9B is switched off to the state of FIG. 9A, the return current flows to the return diode connected in reverse parallel to the switching element C2 as in (2) above. When A2 and B2 switched on in FIG. 9B are switched off to the state of FIG. 9C, the return current flows to the return diodes connected in reverse parallel to the switching elements A1 and B1 as in (2) above respectively.
In the case of FIG. 10, when B1 and C1 switched on in FIG. 10B are switched off to the state of FIG. 10A, the return current flows to the return diode connected in reverse parallel to the switching elements B2 and C2 as in (2) above. When A2 switched on in FIG. 10B is switched off to the state of FIG. 10C, the return current flows to the return diode connected in reverse parallel to the switching element A1 as in (2).
Although the power supply devices of FIGS. 8A to 8F, 9A to 9I, 10A to 10I and 11A to 11I can also be configured by using a composition circuit combining a switching element and a diode, a power supply device can also be produced by combining semiconductor devices in each of which an IGBT domain and a diode element domain coexist in a same semiconductor substrate. The semiconductor device in which the IGBT domain and the diode element domain coexist in the same semiconductor substrate is referred to as a reverse conducting semiconductor device.
(Characteristics Required of Power Supply Device)
If the forward voltage drop of the return diode is large, a constant loss increases and the return diode generates heat. The return diode is required to have a small forward voltage drop. The amount of the voltage drop of the return diode can be decreased by increasing impurity concentrations of an anode and a cathode.
On the other hand, the reverse recovery current flows to the return diode. If the impurity concentrations of the anode and the cathode of the return diode are increased in order to decrease the forward voltage drop, a reverse recovery loss of the return diode increases. If the impurity concentrations of the anode and the cathode are increased, a large quantity of p-type carriers accumulate in the cathode and a large quantity of n-type carriers accumulate in the anode when a forward voltage is applied. When a reverse voltage is applied to the return diode, i.e., when the cathode is connected to a high electric potential side and the anode is connected to a low electric potential side, the p-type carriers within the return diode flow in the direction of the anode, while the n-type carriers flow in the direction of the cathode, thereby resulting in the flow of reverse recovery current. In the case of increasing the impurity concentrations of the anode and the cathode, the quantity of p-type carriers that accumulate in the cathode and the quantity of n-type carriers that accumulate in the anode increase, resulting in the flow of large reverse recovery current. If such a large reverse recovery current flows, a large amount of heat is generated, and electric power is consumed. Moreover, in the case where the current amount exceeds a permissible current of the diode, the return diode is destroyed. By lowering the impurity concentrations of the anode and the cathode, the reverse recovery current can be held to a low level and the reverse recovery loss can be reduced, however, the forward voltage drop as a result becomes large.
Both the constant loss and the reverse recovery loss of the return diode cannot be decreased even by tuning the characteristics of the return diode.
Japanese Patent Application Publication No. 2005-317751 (steady-state 1) discloses a technology that uses a lifetime control to reduce the reverse recovery loss. According to this technology in the aforesaid patent document 1, a low lifetime layer is formed on an impurity injection domain corresponding to the anode and/or the cathode. In the case the low lifetime layer is formed on the cathode, the quantity of p-type carriers that have accumulated in the cathode dissipates in a short period of time after completion of the application of a forward voltage. Similarly, in the case the low lifetime layer is formed on the anode, the quantity of n-type carriers that have accumulated in the anode dissipates in a short period of time. As a result, the reverse recovery current can be decreased, and the reverse recovery loss can be reduced.